Expeller-expander technology is a processing technique that has been available in various forms dating back to the nineteenth century. The present area of concern relates to that area of an elastomer processing sequence (including synthetic and natural elastomers) where the rubber material has been combined with water and now the water is to be removed.
In previously used configurations, two extruders in series have been employed to remove moisture from the rubber. Generally, the first extruder, also referred to as the expeller, squeezes the rubber between a pair of intermeshed screws. This portion of the process is generally able to reduce the moisture content from about 60% to about 15%.
Following the first stage of the drying process by the expeller, the rubber material is passed to a second extruder referred to as an expander for additional drying. This second extruder increases pressure on and consequently the temperature of the rubber, thus creating a super heated liquid. As this super heated liquid is forced through the extruder, again commonly by using screw drive technology, the material is forced through dies or filter screens at the end of the screw where the moisture, or volatile matter, will flash dry.
The flash drying process corresponds to a rapid change in state from liquid to vapor as the supper heated rubber material passes through the die or filter screen and suddenly returns to normal atmospheric pressure while the water temperature may still be significantly higher than 100 degrees Celsius. The energy necessary to produce the flash drying phenomena is transferred to the rubber from the screw drive mechanism in the expander. This transfer of energy is made possible by the resistance of the rubber to exit the expander through the dies. The temperature and pressure on the super heated rubber reach a maximum at the dies, thus for a given screw speed and rubber flow rate, the resistance, and therefore the amount of energy transferred to the rubber, is dependent on the pressure at the head.
The pressure is fixed by the pressure drop induced by the passage of the super heated rubber through the die. In a practical system, there will be a number of dies at the exit point of the expander and thus the pressure will depend on the number of dies, their geometry and aperture size. In previously employed configurations, all of these aspects of the dies were fixed with any one processing sequence. Because the prior art is a fixed and unchangeable configuration, certain production problems have occurred that the present technology addresses and overcomes.
When the super heated rubber goes through the dies, the flash drying process produces decohesion of the rubber thereby creating rubber crumbs that are transported to balers for further processing. The control of the size of these crumbs is one of the aspects effecting good transportation of the rubber through the remaining processing sequences and, consequently, can have an impact on further processing. For example, reduction of conveyor fouling can occur based on production of too small a crumb size. As the currently available technology employs preset die configurations, no capability other than stopping production is available to address issues involving pressure adjustment and crumb size. Moreover, there is no capability for optimizing the overall rubber processing process outside of controlling the expeller-expander screw speed without shutting down production.
While various implementations of extruder-expander technology have been developed, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.